Bear in mind what is quoted for these is "up to" speeds. Performance is still reliant on the standard of your house wiring & what the "signal" has to pass through between the two units. If you are going from one ring main to another expect some loss, especially if there are RCCB's in the circuit.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.
If you are paying for a 350 circuit then I would expect to get full use and speed from it and the only guaranteed way is to use CAT5e/6 cable. In most cases with some ingenuity it can be hidden and it’s the best long term solution whoever your ISP is.
Tudor There are 10 types of people: those who understand binary and those who don't and F people out of 10 who do not understand hexadecimal c1a2a285948293859940d9a49385a2
What you get on your phone is nothing to do with how the powerline will perform. I presume the phone is using WiFi.
How close are the two powerline adapters going to be? In the same room that kit might manage the full speeds of the connection. The speeds drop pretty rapidly with distance though and if not on the same circuit you aren't getting close to full speed.